Over the last few years, group IV hexagonal-diamond type crystals have acquired great attention in semiconductor physics thanks to the appearance of novel and very effective growth methods. However, many questions remain unaddressed on their extrinsic doping capability and on how it compares to those of diamond-like structures. This point is here investigated through numerical simulations conducted in the framework of the Density Functional Theory (DFT). The comparative analysis for group III and V dopant atoms shows that: i) in diamond-type crystals the bulk sites symmetry ($T_d$) is preserved by doping while in hexagonal crystals the impurity site moves towards a higher ($T_d$) or lower ($C_{3v}$) symmetry configuration dependently on the valence of the dopant atoms; ii) for Si and Ge, group III impurities can be more easily introduced in the hexagonal-diamond phase, whose local $C_{3v}$ symmetry better accommodates the three-fold coordination of the impurity, while n-type impurities do not reveal any marked phase preference; iii) for C, both n and p dopants are more stable in the hexagonal-diamond structure than in the the cubic one, but this tendency is much more pronounced for n-type impurities.
Hexagonal diamond has been predicted computationally to display extraordinary physical properties including a hardness that exceeds cubic diamond. However, a recent electron microscopy study has shown that so-called hexagonal diamond samples are in fact not discrete materials but faulted and twinned cubic diamond. We now provide a quantitative analysis of cubic and hexagonal stacking in diamond samples by analysing X-ray diffraction data with the DIFFaX software package. The highest fractions of hexagonal stacking we find in materials which were previously referred to as hexagonal diamond are …
We explore the structural evolution of highly oriented pyrolytic graphite (HOPG) under detonation-induced shock conditions using in-situ synchrotron X-ray diffraction in the ns time scale. We observe the formation of hexagonal diamond (lonsdaleite) at pressures above 50 GPa, in qualitative agreement with recent gas gun experiments. First-principles density functional calculations reveal that under uniaxial compression the energy barrier for the transition towards hexagonal diamond is lower than cubic diamond. Finally, no indication of cubic diamond formation was observed up to >70 GPa.
Superstructures of cubic and hexagonal diamonds (h- and c-diamond) comprising a family of stable diamond-like $sp^3$ hybridized novel carbon allotropes are proposed, which are referred to as U$_n$-carbon where $n \geq 2$ denotes the number of structural layers in a unit cell. The conventional h- and c-diamond are included in this family as members with $n=2$ and 3, respectively. U$_n$-carbon ($n=4-6$), which are unveiled energetically and thermodynamically more stable than h-diamond and possess remarkable kinetic stabilities, are shown to be insulators with indirect gaps of $5.6 \sim 5.8$ eV, …
We have performed ab-initio molecular dynamics simulations to elucidate the mechanism of the phase transition at high pressure from hexagonal graphite (HG) to hexagonal diamond (HD) or to cubic diamond (CD). The transition from HG to HD is found to occur swiftly in very small time of 0.2 ps, with large cooperative displacements of all the atoms. We observe that alternate layers of atoms in HG slide in opposite directions by (1/3, 1/6, 0) and (-1/3, -1/6, 0), respectively, which is about 0.7 Å along the pm[2, 1, 0] direction, while simultaneously puckering by about pm0.25 Å perpendicular to the…
The density of states and the band diagrams were computed for diamond, cubic boron nitrde (cBN), and hexagonal boron nitride (hBN) using a Korringa-Kohn-Rostoker (KKR) scheme to investigate the shift of the Fermi level by impurity-atom doping below 10 at.%. The dopant atoms were B and N for diamond, Be, Si, and C for cBN, and Be and C for hBN. It was found that the Fermi level was located at the valence band maximum or the conduction band minimum in the following seven cases: (i) the B concentration was 0.3 at.% in B-doped diamond, (ii) the N concentration was 0.4 at.% in N-doped diamond, (iii…
The metastable hexagonal-diamond phase of Si and Ge (and of SiGe alloys) displays superior optical properties with respect to the cubic-diamond one. The latter is the most stable and popular one: growing hexagonal-diamond Si or Ge without working at extreme conditions proved not to be trivial. Recently, however, the possibility of growing hexagonal-diamond group-IV nanowires has been demonstrated, attracting attention on such systems. Based on first-principle calculations we show that the surface energy of the typical facets exposed in Si and Ge nanowires is lower in the hexagonal-diamond phas…
In succession to the first paper (arXiv 1406.6204v5), the impurity-atom concentrations when the Fermi levels are either at the valence band maximum (VBM) or the conduction band minimum (CBM) were identified for diamond, cubic boron nitride (cBN), and hexagonal boron nitride (hBN) using the Korringa-Kohn-Rostoker (KKR) scheme using the local density approximation (LDA). In the present paper, the generalized gradient approximation (GGA) was used instead of the LDA for exchange-correlation. The impurity atoms were B and N for diamond, Be, Si, and C for cBN, and Be for hBN; these impurity atoms we…
Conductive and electrostatic atomic force microscopy (cAFM and EFM) are used to investigate the electric conduction at nominally neutral domain walls in hexagonal manganites. The EFM measurements reveal a propensity of mobile charge carriers to accumulate at the nominally neutral domain walls in ErMnO3, which is corroborated by cAFM scans showing locally enhanced d.c. conductance. Our findings are explained based on established segregation enthalpy profiles for oxygen vacancies and interstitials, providing a microscopic model for previous, seemingly disconnected observations ranging from insul…
A new ultra-hard rhombohedral carbon rh-C4 (or hexagonal h-C12) is reported as derived from 3R graphite through crystal chemistry construction and ground state energy within the density functional theory. An extended hexagonal three-dimensional network of h-C12 is formed of C4 tetrahedra alike in h-C4 lonsdaleite (hexagonal diamond). The electronic band structure of rh-C4 is characteristic of insulator with Egap = 4 eV similarly to diamond. From the set of elastic constants a larger value of bulk modulus versus lonsdaleite, and the largest Vickers hardness (HV) versus both forms of diamond wer…
Advanced Technology Research Institute of Taihu Photon Center, School of Physical Science and Technology, Suzhou University of Science and Technology（太湖光子中心先进技术研究院，苏州科技大学物理科学与技术学院）
We predict a new three-dimensional (3D) boron allotrope based on systematic first-principles electronic structure calculations. This allotrope can be derived by substituting each carbon atom in a hexagonal diamond lattice with a B$_{4}$ tetrahedron and possesses the same space group $P6_{3}/mmc$ as hexagonal diamond, hence it is termed as H-boron. We show that H-boron has good stability and excellent mechanical property. Remarkably, we find that H-boron is a topological metal with rich types of spin-orbit-free emergent fermions, including semi-Dirac fermion, quadratic and linear triple-point f…
Diluted magnetic semiconductors possessing intrinsic static magnetism at high temperatures represent a promising class of multifunctional materials with high application potential in spintronics and magneto-optics. In the hexagonal Fe-doped diluted magnetic oxide, 6H-BaTiO$_{3-δ}$, room-temperature ferromagnetism has been previously reported. Ferromagnetism is broadly accepted as an intrinsic property of this material, despite its unusual dependence on doping concentration and processing conditions. However, the here reported combination of bulk magnetization and complementary in-depth local-p…
Nitrogen Vacancy Centers in Hexagonal Diamond Exhibit Long Coherence Times
Pritzker School of Molecular Engineering, The University of Chicago（芝加哥大学普里茨克分子工程学院）；Materials Science Division, Argonne National Laboratory（阿贡国家实验室材料科学部）；Department of Chemistry, The University of Chicago（芝加哥大学化学系）
Extrinsic faulting has been discussed previously within the so called difference method and random walk calculation. In this contribution is revisited under the framework of computational mechanics, which allows to derive expressions for the statistical complexity, entropy density and excess entropy as a function of faulting probability. The approach allows to compare the disordering process of extrinsic fault with other faulting types. The ε-machine description of the faulting mechanics is presented. Several useful analytical expressions such as probability of consecutive symbols in the Haägg…
To study temperature dependent elastic constants, a new computational method is proposed by combining continuum elasticity theory and first principles calculations. A Gibbs free energy function with one variable with respect to strain at given temperature and pressure was derived, hence the full minimization of the Gibbs free energy with respect to temperature and lattice parameters can be put into effective operation by using first principles. Therefore, with this new theory, anisotropic thermal expansion and temperature dependent elastic constants can be obtained for crystals with arbitrary …
BaTiO3 appears in cubic and hexagonal variants, both of which are centrosymmetric. Samples of cubic BaTiO3 are known to exhibit breaking of the centric symmetry locally and globally. It has been proposed that the local symmetry breaking originates in polar regions, the precursors of the ferroelectric phase. Origins of the macroscopic symmetry breaking, which are not well understood, have been previously tentatively correlated with inhomogeneities in the samples, such as strain gradients that may align or redistribute objects such as charged point defects or polar regions making material macros…
In this paper, we report an experimental observation of the large anomalous Hall effect (AHE) in a hexagonal ferromagnetic Fe5Sn3 single crystal with current along the b axis and a magnetic field normal to the bc plane. The intrinsic contribution of the anomalous Hall conductance sigma_AH^int was approximately 613 Ω-1 cm-1, which was more than 3 times the maximum value in the frustrated kagome magnet Fe3Sn2 and nearly independent of the temperature over a wide range between 5 and 350 K. The analysis results revealed that the large AHE was dominated by a common, intrinsic term, while the extrin…
Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN (or hexagonal h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout hexagonal settings featuring extended lattices, the calculation of the hexagonal set of elastic constants, provide results of large bulk moduli i.e. B0(rh-C2) = 438 GPa close to that of diamond, and B0(rh-BN) = 369 GPa close to that of cubic BN. T…
We present an implementation of the Bethe-Salpeter equation (BSE) for core-conduction band pairs within the framework of the projector augmented-wave method. For validation, the method is applied to the $K$-edges of diamond, graphite, hexagonal boron-nitride, as well as four lithium-halides (LiF, LiCl, LiI, LiBr). We compare our results with experiment, previous theoretical BSE results, and the density functional theory-based supercell core-hole method. In all considered cases, the agreement with experiment is excellent, in particular for the position of the peaks as well as the fine structure…
We use molecular simulation to construct equilibrium phase diagrams for two recently introduced model materials with isotropic, soft-repulsive pair interactions designed to favor diamond and simple cubic lattice ground states, respectively, over a wide range of densities [Jain et al., Soft Matter 9 14 (2013)]. We employ free energy based Monte Carlo simulation techniques to precisely trace the inter-crystal and fluid-crystal coexistence curves. We find that both model materials display rich polymorphic phase behavior featuring stable crystals corresponding to the target ground-state structures…
Dielectric breakdown has historically been of great interest from the perspectives of fundamental physics and electrical reliability. However, to date, the anisotropy in the dielectric breakdown has not been discussed. Here, we report an anisotropic dielectric breakdown strength (EBD) for h-BN, which is used as an ideal substrate for two-dimensional (2D) material devices. Under a well-controlled relative humidity, EBD values in the directions both normal and parallel to the c axis (EBD+c & EBD//c) were measured to be 3 and 12 MV/cm, respectively. When the crystal structure is changed from sp3 …
From layered 2D carbon to 3D tetrahedral original allotropes C12 and C18 with physical properties related to diamond: Crystal chemistry and DFT investigations
Simultaneous high pressure x-ray diffraction and electrical resistance measurements have been carried out on a PbO type α-FeSe0.92 compound to a pressure of 44 GPa and temperatures down to 4 K using designer diamond anvils at synchrotron source. At ambient temperature, a structural phase transition from a tetragonal (P4/nmm) phase to an orthorhombic (Pbnm) phase is observed at 11 GPa and the Pbnm phase persists up to 74 GPa. The superconducting transition temperature (TC) increases rapidly with pressure reaching a maximum of ~28 K at ~ 6 GPa and decreases at higher pressures, disappearing comp…
Hexagonal boron nitride (h-BN) is unique among two-dimensional materials, with a large band gap (~6 eV) and high thermal conductivity (>400 W/m/K), second only to diamond among electrical insulators. Most electronic studies to date have relied on h-BN exfoliated from bulk crystals; however, for scalable applications the material must be synthesized by methods such as chemical vapor deposition (CVD). Here, we demonstrate single- and few-layer h-BN synthesized by CVD on single crystal platinum and on carbon nanotube (CNT) substrates, also comparing these films with h-BN deposited on the more com…
Optically addressable spins in widebandgap semiconductors have become one of the most prominent platforms for exploring fundamental quantum phenomena. While several candidates in 3D crystals including diamond and silicon carbide have been extensively studied, the identification of spindependent processes in atomically thin 2D materials has remained elusive. Although optically accessible spin states in hBN are theoretically predicted, they have not yet been observed experimentally. Here, employing rigorous electron paramagnetic resonance techniques and photoluminescence spectroscopy, we identif…
